The present invention relates to an electrode for a compressive and electro-osmotic dehydrator of the filter-press type, wherein liquid containing solids, such as sludge or slurry of service water or sewage, is dehydrated under pressure, and treated electro-osmotically to improve the dehydration. This invention also relates to a method of compressive and electro-osmotic dehydration.
Compressive and electro-osmotic dehydrators of the filter-press type having press diaphragms are shown in Japanese Provisional Patent Publications No. S. 62-125810, No. S. 62-125811, and No. S. 62-125812 of Shinko Pfaudler Co., Ltd. (now Shinko Pantec Co., Ltd.), all laid open on June 8, 1987.
FIGS. 1a-1c herein show steps in the operation of this type of conventional dehydrator, which includes press plates 2a and 2b and a pair of filter cloths 3 between them. The plate 2a has a diaphragm 4 supporting an electrode plate 5a. The other plate 2b has an electrode plate 5b fixed to it.
In the first step shown in FIG. 1a, the press plates 2a and 2b are closed to form a sealed space between them. A liquid containing solids is pumped through passages 6 and squeezed between the cloths 3. An amount of liquid filtrate permeates through the cloths 3 and drains through small holes in the electrodes 5a and 5b and through passages 7. This procedure produces a cake c of sludge and constitutes primary dehydration.
In the second step shown in FIG. 1b, compressed air is supplied to the space between the plate 2a and the diaphragm 4, which swells the diaphragm and compresses the sludge cake c, for example, at 3 kg/cm.sup.2 for 20 minutes. This further dehydrates the cake down to a water content of 80-85%, and constitutes secondary dehydration.
In a later stage of this step, direct current (DC) voltage is applied, for example, at 40 volts for 15 minutes between the electrodes 5a and 5b. This causes electro-osmosis to further promote the dehydration, and constitutes tertiary dehydration. As a result, the water content is reduced to about 50% and the cake volume is greatly reduced.
In the last step shown in FIG. 1c, the press plates 2a and 2b are opened and the cloths 3 are lowered in order to remove the finally dehydrated cake c.
Electrodes for use in such a dehydrator have been composed of various materials comprising metallic plates and sintered carbon plates and various structures which are suited to the different types of dehydrators. Nevertheless, they have had the following problems:
To take full advantage of the utility of compressive and electro-osmotic dehydration, it is of dominant importance to improve the function and durability of the electrodes, which play a major role in the dehydration process, without unreasonably increasing the cost of the electrodes, which account for nearly half of the total cost of the dehydrator. From this viewpoint, typical conventional electrodes have the following problems:
The electrodes composed of conventional metallic plates such as stainless steel and nickel steel have sufficient strength, but they have a short service life because they are ionized by the application of direct current and consumed by elution into the slurry. In addition, eluted metal ions, particularly those of chromium, cause a secondary pollution issue.
The electrodes composed of titanium alloys, plated with noble metals such as platinum, are less consumed by the application of direct current and are functionally excellent; however, they are too expensive for practical use. Moreover, in some processes of electro-osmosis, temporal reversion of polarity proves useful, but this creates a problem since the platinum plating peels off at the time of polarity reversal.
The conventional sintered carbon plates are less likely to be eluted by the application of current; however, they are not satisfactory in terms of service life since their binder coke is selectively corroded by oxidation consumption due to nascent oxygen, thereby causing carbon particles to fall off. Moreover, the sintered carbon plates have a low mechanical strength and are damaged owing to compressive loads during operation. In Japanese Provisional Patent Publication No. S. 60-147208, a pressure-molded electrode made of conductive fibers and a synthetic resin has been proposed in order to resolve this drawback. However, this electrode contains insulating resin, which reduces its conductivity and increases the voltage drop in the electrode. This requires the applied voltage to be increased in order to carry a specified quantity of electricity for electro-osmotic dehydration. Consequently, not only is the service cost increased, but the service life of the electrode is also shortened, which is not economical. In addition, the use of this electrode is not advisable since there is a risk of electric shock in a working environment using both water and high voltages.
Furthermore, when compared with mechanical compression dehydration with a filter press alone, compressive and electro-osmotic dehydration requires lower pressures, but the electrode for this dehydration must withstand at least these pressures as this dehydration also employs compression dehydration. The electrode for an electro-osmotic dehydrator of the filter-press type needs to withstand the bending stress exerted by the diaphragm 4 (FIGS. 1a-1c), and it also must have a sufficient mechanical strength to withstand the thermal stress occurring due to a difference in thermal expansion of the filter plates 2a and 2b and the electrodes 5a and 5b during heat generation (70.degree.-80.degree. C.) in the last phase of dehydration. These requirements cause a problem particularly if the area of compression is large or if the electrode becomes thin due to erosion.
As described above, the usefulness of the electro-osmotic dehydration method is recognized although, because of its drawbacks or problems related to the conventional electrodes at the present time, this method has not yet been established as a sufficiently economical dehydration method.
It is an object of this invention to resolve the problems of the electrode associated closely with the conventional method of compressive and electro-osmotic dehydration and provide an electrode for use in such dehydration which is highly efficient, has high strength and long service life, and can be manufactured comparatively easily at a reasonable cost.
Another object of this invention is to provide a method for electro-osmotic dehydration using such an electrode.